yes, u can easily estimate it by the slope of the I-Vds curve in saturation region. However, if u will try to estimate the lambda parameter in different conditions, for different device sizes, you will notice that the value is not constant, but changes (more or less slightly). This is due to the channel length modulation effect, which in higher level models is described in a more complicated (and accurate) way than simply 1+lambda*Vds. However, the estimation is a good way to try some calculation, especially for not very short channels, to be refined by the simulator.